Space-filling model showing a close-up view of the γ:γ (or D:D) interface between adjacent molecules in the crosslinked D dimer complexed with the peptide GPRP (A). (Adapted and reprinted with permission from Nature [Spraggon G, Everse SJ, Doolittle RF: Crystal structures of fragment D from human fibrinogen and its crosslinked counterpart from fibrin. Volume 389, page 455, 1997].97 Copyright 1997 Macmillan Magazines Limited.) Space-filling (B) and ribbon (C) models of the globular carboxyl-terminal γ chain region, based on the rFbgγC30 structures (γ143-392). Mutation sites at the γ:γ interface are colored orange (B and C), the calcium ion is green, and the peptide GPRP is shown in magenta. The side chains of residues F303 and F304 are shown in white. We hypothesize that these residues may form part of an extended interaction surface between the D and E regions of fibrin. Residues G292, S358, and K380 are also shown; G268 and G292 are indicated by asterisks (C).